A historic decades-old mural was painted over at a New York high school during a repair project

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(CNN)For nearly 80 years, a well-worn but well-loved work of art adorned the third-floor ceiling of a New York City high school.

The mural -- a shimmering night sky with golden-yellow stars -- is a piece of history from the New Deal era when the country commissioned hundreds of artists to brighten up buildings and public spaces during the dark days of the Great Depression.

But now, the beautiful mural "Constellations" is slathered with bright blue paint -- painted over during a repair project at DeWitt Clinton High School in the Bronx,

"It was probably a pretty thoughtless and negligent act that this happened and it never should have happened," said Harvey Smith, the president of the National New Deal Era Preservation Association.

A constellation painted on the ceiling in "Constellations" before it was covered last November

He finds the event troubling because many New Deal murals are priceless pieces of art that carry historical significance.

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"What the New Deal did was bring art to the general population," he told CNN. " And that art often carried messages of everyday life that recorded local history."

Painting over history

"Constellations," was the work of German-born artist Alfred Floegel as part of the nation's Federal Art Project of the 1930s. A second Floegel work covers the walls on either side of the mural. Together, the murals took six years to complete.

The ceiling of a hallway at Dewitt Clinton High School before "Constellations" was painted over

In November, crews painted over "Constellations" as the school repaired damage to its roof, said Doug Cohen, a spokesman for the city's education department.

Frank da Cruz is a national associate at the Living New Deal project at the University of California, Berkeley, which captures the impact of the New Deal on Americans. He said the mural was in pretty bad shape.

"I hope they will restore it and I hope they learn something from having to restore it," da Cruz told CNN

Restoring what was lost

Dewitt Clinton High School, where a historic New Deal-era mural was covered last fall.

Mural restoration is time-consuming and costly. But the need for such restoration is becoming all-too-common.

"The fact that they just painted out something without thinking about it is a real lapse in judgment," said Scott Haskins, an expert in mural restoration who works as head of conservation at the Fine Art Conservation Laboratories in Santa Barbara, California.

To reverse the damage done to "Constellations," Haskins said the acrylic paint on the surface will need to be dissolved carefully to preserve the oil paint underneath.

Bringing back a piece of history

Angel Hernandez, the Bronx County Historical Society's director of programs and external relations, told CNN he is "outraged" tthe mural was painted over.

"It's like going to a museum and painting over a masterpiece because this is what it was," Hernandez said. "How the school just painted over these murals is a blow for Bronx history."

"As a historian and as a person from the Bronx, the paintings should not only be restored and but also touched up as well," he added.

Emails to the school and the superintendent were not returned. The New York Department of Education said it's exploring ways to restore the historic artwork.